Studies are being made on isolating and defining previously unknown modulators of fibrinogen synthesis and on the effects on fibrinogen synthesis of known "inflammatory" hormones. These studies use radioactive iodine and carbon labelled fibrinogen and mathematical models of the fibrinogen system. Studies are also being made on the regulation of the intravascular activity of thrombin by circulating antiproteases, particularly antithrombin III. Initial studies involve the purification and labelling with radioactive iodine (I) of dog antithrombin III (AT3) and following the distribution and metabolism of the I-AT3 in dogs. Similar studies are also being undertaken in man.